The invention relates to a method of controlling the afterpressure variation with time of an injection-moulding machine comprising a plastification and moulding unit, which has a worm operated by a hydraulic pressure drive for transporting the plastified moulding mass into an injection nozzle, and a moulding tool arranged before the injection nozzle for producing a moulding, as well as a process computer which controls the hydraulic pressure drive, which stores a given selectable reference after-pressure variation with time and calculates the temperature values associated therewith and which calculates a new afterpressure variation with time upon variation of a moulding mass temperature Tm measured before the injection nozzle and/or of the tool temperature Tf measured in the moulding tool.
During injection-moulding, the pulverulent or granulated moulding mass is plastified by heating with the aid of a heating means arranged at the worm housing and is pressed during the mould filling stage (mould filling time) into the moulding tool by axial displacement of the worm in the direction of the injection nozzle. When the moulding tool has been completely filled with the moulding mass, the latter is solidified by cooling. After the mould filling stage, the afterpressure stage begins (after-pressure time), in which the pressure produced by the hydraulic pressure drive decreases continuously to the normal pressure. As far as possible, during this stage, no moulding mass will then flow out of the tool and into the tool, respectively. After the afterpressure stage, the demoulding stage begins (demoulding time), in which the tool is opened and the moulding is ejected. At the same time, the worm is moved away from the injection nozzle.
The method mentioned in the opening paragraph of controlling the afterpressure variation with time of an inject on-moulding machine is known from the publication "Prozessrechnereinsatz beim Spritzgiessen" by Dr. Ing. Matzke in "Technischwissenschaftlicher Bericht des Instituts fur Kunststoffverarbeitung an der Rheinisch-Westfalischen Technischen Hochschule Aachen". This method, which is designated as PVT optimization, avoids material movements during the afterpressure stage. The afterpressure time is then calculated for given nominal value of the specific volume at the normal pressure (1 bar). The calculation of the afterpressure time takes place by means of a simplified formula derived from the well known heat conduction equation. The calculated afterpressure time is subdivided into equidistant time sections. Subsequently, at each time data sample the corresponding temperature value is calculated. The afterpressure time can then be determined from a formula establishing a relationship between the specific volume, the pressure and the temperature in the moulding. The calculated temperature and pressure values apply to a given moulding mass temperature and a given tool temperature.
When the moulding mass temperature and/or the tool temperature are varied, a new afterpressure time is calculated. Subsequently, this afterpressure time is subdivided into equidistant time sections and at each time data sample a new temperature value is determined. A new pressure value can then be calculated. The known method comprises a large number of calculation steps so that with the use of a process computer commercially available, when the temperature of the moulding mass or of the tool is varied, an afterpressure variation can be effected not during the proceeding or the next injection-moulding cycle, but only during the nextsubsequent injection-moulding cycle.